The present invention relates to a process for the selective electrochemical dimerization of conjugated dienes to give substantially exclusively dimers having a cyclohexene structure. The invention also relates to an electrochemical catalytic system whereby this dimerization may be effected. According to the process of the invention, 1,3-butadiene produces only 4-vinylcyclohexane, and isoprene gives a mixture of substituted cyclohexenes containing limonenes (methylisopropenylcyclohexenes) and dimethylvinylcyclohexenes.
Selective chemical dimerizations of conjugated diolefins are known, using catalysts or catalytic systems based, for example, on dinitrosyliron chloride. Thus, French Pat. No. 1,502,141 teaches the use of a catalyst consisting of a dinitrosyliron halide, a reducing agent such as an organomagnesium or organoaluminum compound, and a third component such as arsine, stibine, phosphine, ether, sulfide, or amine derivative. U.S. Pat. Nos. 3,655,793 and 3,767,593 disclose a catalyst couple comprising dinitrosyliron halide and an organoaluminum, borohydride or aluminum hydride reducing agent. French Pat. No. 2,225,401 discloses the use of a catalytic couple comprising dinitrosyliron halide and a metal carbonyl.
It is also known, as disclosed in French Pat. No. 2,080,556, that butadiene can be trimerized to 1,5,9-cyclododecatriene or dimerized to 1,5-cyclooctadiene and/or a linear dimer by an electrochemical process, by forming transition metal complexes in situ and operating at a constant voltage and current of electrolysis. There is no teaching in the prior art of conditions under which butadiene could be selectively dimerized in an electrochemical process to form 4-vinylcyclohexene, much less conditions under which such electrochemical dimerizations might be effected with up to 100% selectivity.
A need therefore continues to exist for an electrochemical process for dimerization of conjugated dienes whereby it would be unnecessary to handle compounds which are for the most part toxic, a process which requires very little energy and which produces selectively the desired dimer having a cyclohexene structure.